linux-kernel - Re: [PATCH bpf-next 6/8] arm32, bpf: add support for 64 bit division instruction

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Message-ID: <ZPjgb9LDbAZlw0cp@shell.armlinux.org.uk>
Date:   Wed, 6 Sep 2023 21:26:23 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Puranjay Mohan <puranjay12@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>,
        Yonghong Song <yonghong.song@...ux.dev>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Shubham Bansal <illusionist.neo@...il.com>,
        Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>,
        bpf@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next 6/8] arm32, bpf: add support for 64 bit division
 instruction

On Wed, Sep 06, 2023 at 07:19:50PM +0000, Puranjay Mohan wrote:
> On Wed, Sep 06 2023, Russell King (Oracle) wrote:
> 
> > On Wed, Sep 06, 2023 at 09:29:19AM +0000, Puranjay Mohan wrote:
> >> On Tue, Sep 05 2023, Russell King (Oracle) wrote:
> >> 
> >> > On Tue, Sep 05, 2023 at 09:06:19PM +0000, Puranjay Mohan wrote:
> >> Actually, there can also be a situation where rd[1] != ARM_R0 && rd[1] != ARM_R2,
> >> so should I do it like:
> >> 
> >>  	if (rd[1] != ARM_R0 && rd[1] != ARM_R2) {
> >>  		emit(ARM_POP(BIT(ARM_R0) | BIT(ARM_R1)), ctx);
> >>  		emit(ARM_POP(BIT(ARM_R2) | BIT(ARM_R3)), ctx);      
> >>  	} else if (rd[1] != ARM_R0) {
> >>  		emit(ARM_POP(BIT(ARM_R0) | BIT(ARM_R1)), ctx);
> >>  		emit(ARM_ADD_I(ARM_SP, ARM_SP, 8), ctx);
> >>  	} else if (rd[1] != ARM_R2) {
> >>  		emit(ARM_ADD_I(ARM_SP, ARM_SP, 8), ctx);
> >>  		emit(ARM_POP(BIT(ARM_R2) | BIT(ARM_R3)), ctx);
> >>  	} else {
> >>  		emit(ARM_ADD_I(ARM_SP, ARM_SP, 16), ctx);
> >>  	}
> >
> > Are you sure all four states are possible?
> 
> ohh!
> 
> I just realized that the last else will never run.
> rd[1] can never be equal to both ARM_R0 and ARM_R2.
> Will fix it in V3 as I already sent out the V2.
> 
> I need to learn to leave patches on the list for few days before re-spinning.

The last comment on that is you can pop r0-r3 in one go, rather than
using two instructions.

-- 
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